Energy Storage Systems – UL 9540A

Energy Storage Systems (ESS) are a source of available and reliable power that can provide flexibility to electrical grids during peak usage and assist with load management and power fluctuations. NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, addresses the installation of energy storage technologies and aims to mitigate the hazards related to these systems.

NFPA 855 contains size and separation requirements designed to prevent fire propagation from one ESS to adjacent combustible materials (other ESS, wall assemblies, exposures). These limitations specifically focus on indoor installations in non-dedicated buildings and outdoor installations less than 100 feet from exposures:

  • The maximum stored energy per unit is limited to 50 kWh
  • The separation distance between units and wall assemblies should be a minimum of 3 feet
  • The maximum stored energy of all the ESS units comprising the system is limited to a threshold value based on the battery technology

NFPA 855 also requires most new Energy Storage System (ESS) installations to be listed in accordance with UL 9540, Standard for Safety of Energy Storage Systems and Equipment. UL 9540 provides design, construction, and performance requirements for ESS.

Exceptions in both NFPA 855 and UL 9540 allow for ESS installation with increased stored energy and reduced separation distances. Approvals for larger ESS depend on the results of large-scale fire testing conducted in accordance with UL 9540A, Standard for Test Method for Evaluating Thermal Runaway Fire Propagation in Battery Energy Storage Systems. UL 9540A provides critical information regarding the fire performance of ESS undergoing a thermal runaway event. Fire testing is conducted at the cell, module, and unit level to evaluate the potential for fire spread and toxic gas emissions from the ESS, as well as the performance of the mitigation solutions to limit the consequences of the thermal runaway event.

An extensive amount of data is produced from the UL 9540A testing. Correct interpretation of the results is essential for the AHJ to confidently evaluate the safety of an ESS installation.  We are here to help with the review of UL 9540A test reports and the interpretation of UL 9540A testing data. If you have questions related to ESS fire testing in accordance with UL 9540A, please contact us at

Application of any information provided, for any use, is at the reader’s risk and without liability to Code Red Consultants. Code Red Consultants does not warrant the accuracy of any information contained in this blog as applicable codes and standards change over time. The application, enforcement and interpretation of codes and standards may vary between Authorities Having Jurisdiction and for this reason, registered design professionals should be consulted to determine the appropriate application of codes and standards to a specific scope of work.